The goal of this research is to develop immunotoxins with a new mutant of Pseudomonas aeruginosa exotoxin A which will be used for depletion of lymphocyte subpopulations in vitro. The long-term objective is to make these reagents commercially available to scientists as a method for reproducibly eliminating subpopulations of cells in vitro. The specific aims are: l) prepare immunotoxins with the mutant exotoxin, EFCys, and MAbs to cell surface antigens expressed by lymphocyte subpopulations in mice and humans, 2) characterize the cytotoxicity and specificity of these immunotoxins on cell lines and normal lymphocytes, and 3) directly compare the efficacy of these reagents with currently available techniques, such as antibody and complement-mediated cytotoxicity and "panning." The successful development of these reagents would provide medical researchers with a more effective method than is currently available for depletion of specific cell types.